Automatic fidelity control circuits



April 11i 1939. R, A BRADEN 2,154,G61

AUTOMATIC FQIDELITY CONTROL CIRCUITS original Filed July 29, 1953 Il 70 AMPZIF/ER INVENTOR. RENE A.BRADEN BY 71@ Z f f MM ATTORNEY.

Patented Apr. 11, 1939 AUTOMATIO FIDELITY CQNTROL CIRCUITS Rene A. Braden, Collingswood, N. J1., assignorto Radio Corporation of America, a corporation of Delaware Original application Julyv 29,v 1933,. Serial No. 682,743. Divided and this. application January 9, 1935, Serial No. ,l,005

12 Claims. (Cl. 179-171) My present invention relates to fldelity control arrangements for radio frequency signaling systems, and more particularly to automatic fidelity control circuits for radio receivers, and is a diindicated dagrammatically several arrangements for carrying my invention into effect.

In the drawing,

Fig. l shows an embodiment of the invention vision, of application Serial No. 682,743, led July as applied to a band passnetwork,

V29, 1933 now Patent No. 2,088,229. Fig. 2 shows amodication of the invention Itis one of the primary objects of my present Whichis applied tothe tuned input circuit of an invention to provide a method of automatically amplifier. controlling the selectivity of. arradio frequency Referring 110W t0 the accompanying drawing signalling system whereby the system is more wherein like reference characters in the two selective on weak signals than on strong ones. ignreS designate Similar circuit` elements, there Another important object of the present inisshown, in Fig. 1in highly C0I1Venti0ne1 ferm, ventionis to provide an automatic delity control e radici receiving System einbOdying One fOrm 0f for a radio receiver of the type wherein the my inVentiOn- BY0ed1y,it1nay hestetedthetthe resonance. curve of a tunable network. is autoreceiving circuit shown in Fg- 1 includes means matically varied. in response to the variation in fOr Varying the selectivity 0f e receiving Set, end amplitude 0f the received Signal energy, which means comprises a signal actuated con- Another important object of the present inti'Ol tube- 1 l vention is to-provide an automatic selectivity con- In Fig 1 there iSShOWn en arrangement fOr Setrol circuit for e, radio receiver wherein the reecuring Yautomatic selectivity control wherein a tied signal` energy of the receiver is utilizedv to demping ectcn 0n One 0.5 the tuned CilCllits be' vary the high frequency response characteristic tween amplifiers end 'if iS Secured by the utili- Of e resonant Circuit preceding., the receiver ree zation of Va feedback circuit. Between the screen tier, grid amplifiers E `and 'I' there is connected a Still another object of the present invention is coupling netwerk Which COInlJriSeS @Deir 0f C011- to provide a fidelity control arrangement for a Died tuned circuits 350 and 3i- The tuned Cirradio receiver, theletter operating on abend pass cuit et includes in e Series arrangement a coil network preceding a rectier in such a manner 322 the tuning Cen-denser 33, and the coupling that `the resonance Curve ofthe networkis autocoil 34. The tuned circuit 3| includesin a similar matically variedlto have a` single resonancepeak series arrangement the Geil 32', the tuning c0nwhen weak, or distant, station Signals are received, denser 33', and the 00111311118 Conil 34'- The Insigand a double peak when Strong, or local staf neticv coupling M1 between coils 34 and 34' is tions are received. less thanAC-ritical.

Another object of the present invention is to The rectifier 3v following amplifier 7'? is shown 35 provide an automatic selectivity control circuit as of the. biased detector type A resistor R 1s for a receiver wherein a rectifier automatically dtsposedm-ths gl'flunded Cathode leed 0f tube 8; regulates the amount of damping introduced into the Source B prot/101mg the positlve Potetital OT a preceding tuned circuit, the damping being the plate of the detector tube. The aud1o comincreased when receiving strong signals, and beponntof detected lgnats 1S fed to any desired 40 ing decreased when receiving weak signals. and?? frequency utlhzatlotl network' The 5?' Still other objects of the present invention are lectlvlty Contrl tube 35 1s of the Screengufi to improve generally the efficiency of radio retype and has lts .plate mtcult Coupled to circuit ceiving systems, and particularly to provide auto- 3u' byszthe nllagnetl? C Ouphrlg- M between 3.0115 36, matic fidelity control circuits for radio receivers t and T e Ope1a-t1ng drect' Culrent bias fol which are not only emcent and reliable in 0p the control tube 35 1s dependent on the voltage oi' xed bias` source` 38 and the variable potential eration, but econom1cally constructed and assemdrop across that portion *of resistor R between bled 1n radlo recelvers' ground and, adjustable tap 5U. The signal grid Th? I lovel features Winch I beheve to b e Char' of tube 35 is connected to the negative terminal afztenstlc Pf my mventlon aire set form? m p al" of bias source 38 through, filter resistor R1; ticularity in the appended claims, the invention the Cathode of tube 35 being grounded Signal itself, hOWeVer. eS t0 100th its OrganiZatiOn and voltage is impressed upon the signal grid of tube method of operation will best be understood by 35 through condenser 3l. The coils 32 and 36 reference to the following description takenv in are so poled that the signal voltage, repeated 55 connection with the drawing in which I have through tube 35, is impressed on circuit 30 in degenerative phase. That is to say, the effect of tube 35, as its gain increases, is to increase the damping of network 30-3I. This results from the degenerative phase of the signal voltage fed back, through coil 36, to circuit 30.

The tube 35 feeds back signal voltage to the tuned circuit 30, and the feed-back polarity is opposite to that required for oscillation. IIC-Iencc,4

there is a tendency to reduce the output of the receiving system as the received signal amplitude increases. The reduction of output is greatest at resonance, and therefore the over-all selectivity curve of the system is flattened into a band pass curve. By regulating the bias of tube 35 the sharpness of the selectivity curve can be altered. This is accomplished automatically by the signal so that the selectivity is inversely proportional to the signal strength. In other words, the auX- iliary control tube 35 connected to the network between tubes 6 and 1', which network may be operating at intermediate frequency or at a much higher frequency, produces reversed feedback, which in turn reduces the gain and pushes down the center of the resonance curve so as to make it broad and flat, or double-peaked. The bias on the control grid of tube 35 controls the reversed feed-back; accordingly, if the bias is high there is no feedback and the coupling network tunes sharply and has a normal gain.

In the absence of received signals the network 30-3I has a sharp single peaked resonance curve; the coupling M1 being less than critical. Again, the gain of tube 35 will be a minimum, in this no-signal condition, due to the negative bias from source 38. Hence, the damping action on network Sil- 3L caused by the effect of the degenerative coupling M, will be a minimum, since substantially no feedback occurs through tube 35 when the negative bias is high.

Vvhen signals of weak amplitude, as from distant stations, are received, the bias on the grid of tube 35 will be high; this following from the fact that the negative bias source 38 overcomes the effect of the voltage developed in resistor R between ground and the grid tap 50. As the signal amplitude increases, this voltage across the portion of resistor R between ground and the grid tap from tube 35 increases; thus overcoming the effect of the negative bias from source 38 and increasing the gain of tube 35 as well as the reversed feedback effect.

A modification of the arrangement shown in Fig. 1 is represented in Fig. 2. The amplifier tube l is connected across the tuned circuit 40, the latter including the coil 4|, the resistor Rz and the tuning condenser 42. The function of the resistor R2 is to broaden the tuning of the circuit 40. The control tube 35 provides regeneration which sharpens the resonance curve and increases the gain. The amount of regeneration is controlled by the control grid bias, or screen voltage if desired, of tube 35', the latter in turn depending on the signal strength.

In this form of the invention the resistor R', which develops the signal-responsive bias for tube 35', is connected between ground and the negative terminal of detector plate voltage source B. The cathodes of tubes 35 and 8 are at ground potential, and tap 50 is connected from the signal grid of tube 35 to a desired point on resistor R through filter resistor R1. The coil 36', in the plate circuit of tube 35', is magnetically coupled to coil 4 I, as at M2, in regenerative phase. Signal Voltage is impressed on the signal grid, of tube 35 through condenser 31. Normally, and

in the absence of signals, the gain of tube 35' is a maximum, since the voltage drop across resistor R is then a minimum. Therefore, the regenerative feedback will be a maximum, and the effect of damping resistor R2 will be compensated for to an extent suiicient to impart a sharp, single peak resonance curve characteristic to circuit 40.

With increasing signal strength the bias of tube 35 must be increased to reduce the regeneration. Several stages of this kind can be used in cascade. The function, then, of tube 35 of Fig. 2, is to regenerate in the tuned circuit 40, which may be a radio frequency or intermediate frequency stage which has high damping, so as to reduce the effective dampingand, thus sharpen tuning and increase gain when weak signals are being received. When strong signals are being received the regenerative feed-back to the circuit 40 is reduced, and hence the effective resonance curve of circuit 43 is flattened thereby increasing the fidelity of reception.

While I have indicated and described several systems for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular circuits shown and described, but that many modifications may be made without departing from the scope of my invention as set forth in the appended claims.

What I claim is:

1. In a receiving system, a pair of amplifier tubes, a tuned circuit coupling said tubes, said tuned circuit being resonant to a desired signal frequency, a rectifier, means for impressing signal frequency energy upon said rectifier, and means, responsive to variations in potential of the direct current component of the rectified output of said rectifier, for adjusting the response characteristic of said tuned circuit, said means including a device for regulating the damping of said tuned circuit, said damping regulating means consisting of a tube having its anode circuit reactively coupled to said tuned circuit to impress on the latter signal energy repeated through the said last tube, and its control grid connected to the output circuit of said rectifier, the constants of the last tube being such as to adapt it solely for non-rectifying signal repeater action, said coupling being such that the damping of said tuned circuit is regulated, and a signal frequency path connected between the control grid of said damping regulating tube and said tuned circuit for impressing signal energy upon said regulating tube.

2. A band pass network of the type comprising aA pair of resonant circuits, each circuit being tuned to the operating carrier frequency, the circuits being coupled to provide a normally sharp, single peak resonance curve characteristic, reversed carrier energy feedback means connected in cyclic sequence with said circuits for damping the network sufficiently to impart to it a wide resonance curve characteristic, and means, responsive to signal amplitude variations, for automatically regulating the operation of said means.

3. A band pass network of the type comprising a pair of coupled resonant circuits having a normally sharp resonance curve characteristic, each circuit being tuned to the operating carrier frequency, reversed carried energy feedback means connected in cyclic sequence with said circuits for damping the network sufliciently to impart to it a wide resonance curve characteristic, and a signal rectifier connected to said reversed feedback means to render the latter inoperative when weak signals are transmitted through the network.

4. In a signal receiving system including a tube provided with an input circuit tuned to a desired signal frequency, damping signal feedback means operatively associated with the tuned circuit to impart a resonance curve characteristic to it which is relatively broad for strong signals, and additional means responsive to a decrease in signal amplitude, directly controlling said feedback means in a sense substantially to minimize the feedback efficiency thereof whereby said characteristic is automatically made sharp for weak signals.

5. In a signal receiving system including a tube provided with an input circuit tuned to a desired signal frequency, reversed signal feedback means operatively associated with the tuned circuit to impart a resonance curve characteristic to it which is relatively broad for strong signals, and additional means, responsive to a decrease in signal amplitude, for substantially minimizing the effect of the rst means whereby said characteristic is automatically made sharp for weak signals.

6. In a signal receiving system including a tube provided with anV input circuit tuned to a desired signal frequency, signal feedback means operatively associated with the tuned circuit to impart a resonance curve characteristic to it which is relatively broad for strong signals, and additional means including a signal rectifier tube connected to the feedback means, responsive to a decrease in signal amplitude, for substantially minimizing the effect of the first means whereby said characteristic is automatically made sharp for weak signals.

7. A band pass network of the type comprising a pair of coupled resonant circuits having a normally sharp resonance curve characteristic, each circuit being tuned to the operating carrier frequency, and reversed feedback means for damping the network sufliciently to impart to it a wide resonance curve characteristic, said means comprising a tube having its input and output circuits reactively coupled to said network, and

the output circuit being connected in the degenerative phase with said network.

8. A method cf operating a signal transmission network of the type including at least two coupled circuits arranged in cascade and tuned to the same signal carrier frequency, which consists in impressing signals of a desired amplitude, and of said frequency, upon the network, and feeding back said signals from the output of the network to the input in degenerative phase to maintain substantially uniform the amplification of the modulation frequencies of said carrier frequency energy transmitted through said network at said desired amplitude.

9. A method of operating an amplifier provided with a tuned input circuit having a normally sharp resonance curve characteristic, which consists in impressing signals upon the input circuit, feeding back signals without rectification for re-impression upon the input circuit substantially to widen said characteristic, and automatically controlling solely the feedback intensity in response to signal amplitude variations to regulate said characteristic.

10. A method of operating an amplifier having a tuned input circuit provided with a predetermined resonance characteristic which consists in impressing signals upon the input circuit, feeding back signals in degenerative phase for re-impression upon the input circuit, and automatically controlling solely the feedback intensity in response to signal amplitude variations to regulate the shape 'of said characteristic.

l1. In a radio receiver having an amplifier, a feedback circuit in said amplifier, a relay tube connected to said circuit and controlling said feedback, and means responsive to the amplifier output energy for controlling said relay tube in accordance with signal strength, whereby the selectivity of said amplier is altered.

12. In a radio receiver having an amplifier, a degenerative feedback circuit in said amplifier, a relay tube connected in said circuit and controlling said degenerative feedback, and means for controlling said relay tube in accordance with signal strength, whereby the selectivity of said amplifier is altered.

RENE A. BRADEN. 

